Tectonics

Late Cenozoic extension and crustal doming in the India-Eurasia collision zone: New thermochronologic constraints from the NE Chinese Pamir

Authors


Corresponding author: R. C. Thiede, Institut für Erd- und Umweltwissenschaften, Universitaet Potsdam, DE-14476 Potsdam, Germany. (thiede@geo.uni-potsdam.de)

Abstract

[1] The northward motion of the Pamir indenter with respect to Eurasia has resulted in coeval thrusting, strike-slip faulting, and normal faulting. The eastern Pamir is currently deformed by east-west oriented extension, accompanied by uplift and exhumation of the Kongur Shan (7719 m) and Muztagh Ata (7546 m) gneiss domes. Both domes are an integral part of the footwall of the Kongur Shan extensional fault system (KES), a 250 km long, north-south oriented graben. Why active normal faulting within the Pamir is primarily localized along the KES and not distributed more widely throughout the orogen has remained unclear. In addition, relatively little is known about how deformation has evolved throughout the Cenozoic, despite refined estimates on present-day crustal deformation rates and microseismicity, which indicate where crustal deformation is presently being accommodated. To better constrain the spatiotemporal evolution of faulting along the KES, we present 39 new apatite fission track, zircon U-Th-Sm/He, and 40Ar/39Ar cooling ages from a series of footwall transects along the KES graben shoulder. Combining these data with present-day topographic relief, 1-D thermokinematic and exhumational modeling documents successive stages, rather than synchronous deformation and gneiss dome exhumation. While the exhumation of the Kongur Shan commenced during the late Miocene, extensional processes in the Muztagh Ata massif began earlier and have slowed down since the late Miocene. We present a new model of synorogenic extension suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting Alai slab localize extensional upper plate deformation along the KES and decouple crustal motion between the central/western Pamir and eastern Pamir/Tarim basin.

1 Introduction

[2] Collisional orogens are often characterized by coeval thrusting, strike-slip faulting, and normal faulting [Tapponnier et al., 1981b; Armijo et al., 1986; England and Houseman, 1989; Ratschbacher et al., 1991]. Structural observations and sedimentological evidence from older mountain belts suggest that this phenomenon is a hallmark of late-stage orogenic evolution [Royden and Burchfield, 1987; Vanderhaeghe et al., 1999; Hodges et al., 2001; Teyssier and Whitney, 2002]. However, the specifics of the spatiotemporal development of extensional processes in continental collision zones are less well known due to ambiguous chronological data on the affected tectonostratigraphic units, the vast extent of the orogenic systems, and the poor preservation of corresponding structures.

[3] The Pamir collisional orogen constitutes the northwestern indenter of the India-Eurasia collision zone and is a convex northward mountain belt that forms an integral part of the Himalaya-Tibet orogen [e.g., Burtman and Molnar, 1993]. The Pamir is seismically active and accommodates a significant amount of the India-Eurasia convergence [Reigber et al., 2001; Zubovich et al., 2010]. Present-day crustal kinematics based on earthquake focal mechanisms, geodetic measurements, and neotectonic observations show that most of the Pamir is characterized by thrusting and strike-slip faulting along its northern and western margins, with predominantly normal and strike-slip faulting dominating the plateau-like interior and the eastern flank of the orogen [Burtman and Molnar, 1993; Strecker et al., 1995; Reigber et al., 2001; Strecker et al., 2003; Mohadjer et al., 2010; Zubovich et al., 2010].

[4] In contrast to the neighboring extensive Tibetan Plateau and despite the general plateau characteristics, the Pamir is deeply dissected, with extensive domains of exposed crystalline rocks. Shortening and transpressional deformation throughout the Pamir have been documented since the Miocene [Tapponnier and Molnar, 1976; Hamburger et al., 1992; Burtman and Molnar, 1993; Coutand et al., 2002; Schwab et al., 2004]. The rocks forming the western and central Pamir domes were affected by north-south directed shortening, crustal underthrusting, stacking, and thickening during the late Oligocene to early Miocene. Maximum crustal thickness in this region was likely achieved in the early Miocene, as evidenced by early Miocene peak prograde metamorphism and leucogranite crystallization [Schwab et al., 2004; Robinson et al., 2007; Schmidt et al., 2011; K. Stübner et al., The giant Shakhdara migmatitic gneiss dome, Pamir, India–Asia collision zone, II: Timing of dome formation, submitted to Tectonics, 2013 (hereafter, Stübner et al., submitted, 2013a)]. Exhumation of rocks from mid-crustal depths occurred subsequently during north-south extension and resulted in the formation of large gneiss domes with medium- to high-grade metamorphic basement units bounded by extensional detachment zones [Schmidt et al., 2011; Stübner et al., submitted, 2013a; Stübner et al., The giant Shakhdara migmatitic gneiss dome, Pamir, India–Asia collision zone, I: Geometry and kinematics, submitted to Tectonics, (hereafter, Stübner et al., submitted, 2013b)].

[5] Despite the overall contractile regime in the Pamir, north-south striking transtensional fault systems started to evolve during the early late Miocene in the northeastern region of the orogen and cross-cut preexisting structures [Arnaud et al., 1993; Brunel et al., 1994; Strecker et al., 1995; Robinson et al., 2004, 2007]. These younger faults are characterized by an asymmetric distribution of NNW-SSE to NW-SE striking normal faults associated with strike-slip faults, suggesting ongoing east-west extension in the northeast Pamir. The most prominent tectonic features associated with this phase of extension are the 250 km long Kongur Shan extensional fault system (KES) and the growth of Kongur Shan and Muztagh Ata domal massifs (see Figure 2). Normal faulting has been significant along the KES, as documented by exhumed high-grade metamorphic rocks (~650°C to 700°C and 8 kbar for Kongur Shan, ~700°C to 750°C and 9–10 kbar for Muztagh Ata) [Robinson et al., 2004, 2007] forming the high topography (elevation of >7500 m) in its footwall and a high-elevation (>3 km) intermontane basin in its hanging wall. Despite these advances in our understanding of mountain building in the Pamir, the timing, rates, deformation mechanisms, and details of northward indentation uplift and extensional processes remain insufficiently constrained. Neither is it clear why there are such pronounced differences in crustal deformation processes between the Pamir and the Tibetan Plateau.

[6] The rich structural inventory of different fault generations and the change in kinematic regimes from shortening to extension make the Pamir an ideal site to analyze shortening and extension processes in the early stages of a collisional orogeny and the causes of structural disparities in the long-term evolution of orogenic plateaus. In addition, a better understanding of the crustal processes driving the evolution of the Pamir orogen will help in evaluating the causes of temporal and spatial variations in gneiss dome formation, the relationship between crustal thickening and extension, and the impact of different subduction processes on the regional structural evolution during continental collision processes in the interior of Asia [e.g., Schwab et al., 2004; Robinson et al., 2007; Robinson, 2009; Cowgill, 2010; Robinson et al., 2010; Schmidt et al., 2011; Sobel et al., 2011; Sippl et al., 2013; E Sobel et al., 2013; Stübner et al., submitted, 2013a, 2013b].

[7] In this study, we present new apatite fission track (AFT), zircon (U-Th)/He (ZHe), and 40Ar/39Ar mica thermochronologic data to provide constraints on dome formation in the footwall of the KES and furnish information on the temporal variation in the evolution of different KES fault segments along the strike. We combine this information with published thermochronologic data to determine the role of the KES in accommodating deformation since the onset of E-W extension in the northeast Pamir and assess different models of dome formation, followed by a discussion of the localization of normal faulting in the northeastern sector of the orogen in light of the recent geodynamic evolution of the Pamir indenter.

2 Tectonic Setting

2.1 Cenozoic Tectonic Framework of the Pamir

[8] Continued northward indentation of India since 55–50 Ma has caused the reactivation of preexisting crustal weak zones, underthrusting, and stacking of crustal blocks within the Pamir, forming an orogen with a hot, weak crust of approximately 70 km thickness [Burtman and Molnar, 1993; Mechie et al., 2012]. The Pamir, as an integral part of the Pamir-Tien Shan convergence zone, forms the northwestern continuation of the Tibetan plateau and interacts with the Tarim Basin to the east, the Tadjik Basin to the west, and the Tien Shan to the north. The Tadjik and Tarim basins comprise portions of a formerly contiguous sedimentary basin [Hamburger et al., 1992; Burtman and Molnar, 1993; Coutand et al., 2002]. Due to the continuing northward indentation of the Pamir, the Alai Valley along the northern rim of the orogen is the areally limited vestige of this basin. Based on early Soviet era mapping, the Pamir can be divided into four major tectonostratigraphic terranes bounded by suture zones (for a review, see Burtman and Molnar, [1993]), which are stretched and bent into belts parallel to the convex northward arc of the Pamir salient; these sectors of the orogen have been correlated with terranes in Tibet [Yin and Harrison, 2000; Schwab et al., 2004; Robinson et al., 2012], emphasizing the common evolution of both areas in the course of India's indentation into Eurasia. Despite these first-order similarities, the Cenozoic upper crustal evolution of the Pamir differs significantly from that of Tibet with respect to (a) strong variation in the amount of internal crustal shortening, (b) the degree of subduction of the continental lithosphere, (c) stacking of crustal blocks, and (d) the formation of extensional basement domes [Schmidt et al., 2011]. For example, in contrast to Tibet, the Pamir is characterized by ubiquitous migmatite-cored gneiss domes. In addition, in Tibet, only ~300–500 km of Cenozoic N-S shortening has been recognized [DeCelles et al., 2002; Kapp et al., 2005], whereas ~600–900 km of N-S shortening has been calculated for the Pamir orogen [Burtman and Molnar, 1993; Schmidt et al., 2011].

[9] Beneath the northern Pamir, geological and seismic evidence points to the existence of a south-dipping, intermediate-depth seismic zone, which has been linked with the south-dipping crustal-scale Main Pamir Thrust (MPT) [Roecker, 1982; Hamburger et al., 1992; Burtman and Molnar, 1993; Pegler and Das, 1998]. This crustal scale fault system is responsible for the uplift of the North Pamir-Trans-Alai range, while another thrust system to the north is associated with active seismogenic deformation and uplift of Neogene to Quaternary sedimentary units [Nikonov et al., 1983; Burtman and Molnar, 1993; Arrowsmith and Strecker, 1999]. It has been interpreted that by northward indentation and translation over the eastern continuation of the Tadjik basin the northern Pamir margin has accommodated ~300 km of shortening, coupled with the south directed subduction of Eurasian continental lithosphere, also known as the Alai slab [Burtman and Molnar, 1993, and references therein; Fan et al., 1994; Coutand et al., 2002; Negredo et al., 2007; Sippl et al., 2013].

[10] Deformation of the convex northward Pamir has been interpreted as a regional salient, flanked by major lateral shear zones, with the Quetta-Chaman and Darvaz fault systems in the west and the Karakorum fault system (KFS) and Kashgar-Yecheng transfer system (KYTS) in the east (see Figure 1) [Molnar and Tapponnier, 1975; Tapponnier and Molnar, 1979; Le Pichon et al., 1992; Sobel and Dumitru, 1997; Cowgill, 2010]. Alternatively, the narrow northward convexity of the Pamir indenter was attributed to slab rollback of the narrow, curved south-dipping Alai slab beneath the Pamir [Sobel et al., 2013]. In this view, the subducted Alai slab is bounded on the east by the dextral KYTS, which is the surface trace of a crustal tear fault of this slab. Such a structure represents a subduction-transform edge propagator or STEP fault [Govers and Wortel, 2005].

Figure 1.

Shaded relief map of the Pamir and adjacent areas in the Himalaya-Tibet realm, with principal active faults and tectonic boundaries indicated [Molnar and Tapponnier, 1975; Burtman and Molnar, 1993; Schwab et al., 2004; Cowgill, 2010].

[11] Since the late Oligocene to early Miocene, relative crustal motion between the indenting Pamir and the Indian subcontinent, Tibet and Tarim has been accommodated along the 800 km long Karakorum fault [Ratschbacher et al., 1994; Searle et al., 1998; Leloup et al., 2011] and the 280 km long KYTS. While some authors have interpreted the KES as a splay at the northern termination of the KFS [Tapponnier and Molnar, 1979; Arnaud et al., 1993; Ratschbacher et al., 1994; Chevalier et al., 2011], Robinson et al. [2007], Robinson [2009] showed that slip along the KFS does not extend north of the East Pamir shear zone and the Rushan Pshart zone in central Pamir. This view is supported by the lack of lateral offset between gneiss domes of the central Pamir on both sides of the KES [Robinson et al., 2007]. Deformation along the eastern Pamir margin has been accommodated along the KYTS, a major transpressional dextral transfer system, which links north directed thrusting along the MPT to the west with north directed shortening in the Western Kunlun Shan, the northwestern termination of the Tibetan Plateau [Tapponnier et al., 1981a; Brunel et al., 1994; Sobel and Dumitru, 1997; Sobel, 1999; Cowgill, 2010]. Approximately 280 km of displacement have been inferred for the KYTS since the Oligo-Miocene times, but fault motion appears to have slowed down since the late Miocene [Sobel et al., 2011]. This assessment agrees well with GPS measurements, which indicate that the Tarim Basin and the East Pamir are moving together in the same direction and with similar velocity [Reigber et al., 2001; Mohadjer et al., 2010; Zubovich et al., 2010].

2.2 Evolution of Normal Faults and Basement Domes

[12] Gneiss domes are very common in southern, central, and northeastern Pamir and comprise about 20% of the surface outcrops (Figure 2) [Schmidt et al., 2011]. The domes are dominated by metasedimentary sequences but also include Paleozoic orthogneisses and Mesozoic and Cenozoic granitoids, in places as young as Mio-Pliocene [Hubbard et al., 1999; Schmidt et al., 2011; Stübner et al., submitted, 2013a]. Peak prograde Barrovian metamorphism (22–17 Ma), leucogranite crystallization (22–19 Ma), and structural and metamorphic fabrics indicate that the central Pamir crust was significantly thickened during N-S contraction. Dome formation began in the late Oligocene to early Miocene during N-S oriented extension associated with syntectonic to post-tectonic plutonism [Schmidt et al., 2011; Stübner et al., submitted, 2013a, 2013b]. Dome exhumation ceased in central Pamir by the middle Miocene but continued in the south until the late Miocene to Pliocene [Hubbard et al., 1999; Stübner et al., submitted, 2013a].

Figure 2.

Simplified tectonic setting of the Pamir showing regional distribution of large gneiss domes and the location of Miocene granitic intrusions [compiled from Hamburger et al., 1992; Burtman and Molnar, 1993; Strecker et al., 1995; Coutand et al., 2002; Robinson et al., 2004; Schwab et al., 2004; Robinson et al., 2007; Schmidt et al., 2011]. Topography is shown as background shaded relief map. Red arrows show selected GPS vectors modified after Zubovich et al. [2010] and Mohadjer et al. [2010], whereas blue arrows denote data from Yang et al. [2008].

[13] In contrast to the central and southern Pamir domes, the northeast Pamir domes record a late Miocene transition from dominantly north-south to east-west oriented extension during their formation, which has continued until the present day. East-west directed normal faulting along the west-southwest-dipping faults of the KES began at approximately 7 Ma [Robinson et al., 2004; Robinson et al., 2010]. Normal faulting and seismicity record ongoing east-west extension in the Lake Karakul area in the north central Pamir [Strecker et al., 1995; Amidon and Hynek, 2010] and in the southern Pamir (Stübner et al., submitted, 2013b). This extensional tectonism is consistent with geodetic measurements and neotectonic observations across the Pamir-Tien-Shan convergence zone [Burtman and Molnar, 1993; Strecker et al., 1995; Reigber et al., 2001; Mohadjer et al., 2010; Zubovich et al., 2010]. Zubovich et al. [2010] estimated a rate of 9 ± 2 mm a−1 of east-west extension across the Pamir, while thrusting along the northern Pamir deformation front continues to accommodate ~13 ± 4 mm a−1 of north-south directed shortening.

[14] Various models have been proposed to explain the onset of synorogenic east-west extension and kinematic changes in the eastern Pamir. These models include (a) radial overthrusting along the MPT associated with crustal-scale folding and normal faulting [Brunel et al., 1994], (b) radial thrusting along the Main Pamir thrust and/or oroclinal bending of the Pamir salient [Strecker et al., 1995], or (c) a transtensional bend in the right-slip Karakorum fault, linking late Cenozoic extension in the Pamir with extensional structures in southwest Tibet [Le Pichon et al., 1992; Ratschbacher et al., 1994; Murphy et al., 2000]. Alternatively, (d) a new model suggests that uplift of the KES footwall is due to both thermal effects of toroidal mantle corner flow and the density contrast between the Pamir and Tarim blocks localized by the KYTS STEP fault [Sobel et al., 2013]. Constraining the spatiotemporal evolution of the KES will thus help elucidate when the active east-west oriented extension was established and reconstruct the regional tectonic history of the Pamir-Tien-Shan convergence zone.

2.3 Kongur Shan Extensional Fault System and the Kongur Shan and Muztagh Ata Domal Massifs

[15] From north to south, the KES comprises four fault segments: (a) the King Ata Tagh fault northwest of the Kongur Shan massif; (b) the Kongur Shan and Muztagh Ata normal faults; (c) the Tahman normal fault; and (d) the Tashkorgan normal fault (Figures 3 and 5). We show that these segments vary significantly in relief (Figure 3b), temporal and spatial evolution of fault displacement, and the grade of metamorphic rocks exposed in their footwalls. Footwall rocks record west to southwest-plunging mineral stretching lineations along the Kongur Shan and Muztagh Ata fault segments. In contrast, in the northern King Ata Tagh segment fault, striations plunge southward. Kinematic indicators show a top-to-the-west or southwest sense of shear in the Kongur Shan and Muztagh Ata segments and top-to-the-south indicators in the King Ata Tagh range [Arnaud et al., 1993; Brunel et al., 1994; Robinson et al., 2007]. Whereas mylonitic schist, high-grade gneisses, and deformed granites in the Muztagh Ata massif began to exhume in the middle Miocene, the exhumation of the Kongur Shan massif did not begin until the late Miocene [Arnaud et al., 1993; Robinson et al., 2004].

Figure 3.

(a) New ZHe and AFT cooling ages are plotted on a geological and structural map modified after Robinson et al. [2012]. The new ages suggest significant lateral variations in cooling along different fault segments along the KES, where young cooling ages are obtained along the Kongur Shan and northern Muztagh Ata normal fault segments; older ages are observed both to the northwest along the Muji normal fault segment and to the south along the southern Muztagh Ata and Tahman normal fault segments. Compare also with the regional compilation in Figure 7. Background shaded relief is calculated from the SRTM (90 m) data set. (b) Topographic relief was calculated using a 5 km moving circle across the SRTM (90 m)-derived digital elevation data set. Transparent white rectangles indicate the location of the 15 km wide topographic and relief swath profiles presented in Figure 6b.

[16] The Kongur Shan massif extends from the northern termination of the Muztagh Ata culmination near the Kuksay glacial valley to the Gez River. Foliation in the schists and granites as well as compositional banding in the gneisses define a north to northwest-trending, north northwest-plunging dome, with the western flanks dipping subparallel to the ~40° west-dipping KES. The eastern flank is bounded by the steeply east dipping (>70°), right-slip Gez fault, formerly a subhorizontal shear zone prior to the onset of doming and east-west extension [Robinson et al., 2007]. Thermobarometry (8 kbar, 650°C–700°C) and geochronology studies by Robinson et al. [2004, 2007, 2010] suggest a constant exhumation rate on the Kongur Shan normal fault on the western flank of the massif of 4 ± 1 mm a−1 since the onset of extension at ~7 Ma (40Ar/39Ar K-feldspar cooling ages and multidomain modeling).

[17] The Muztagh Ata massif extends from the Tashkorgan River gorge in the south to the southern termination of the Kongur Shan massif in the vicinity of the Kuksay glacial valley. Foliations define a large, upright to slightly overturned, south-vergent antiform with a west-plunging domal structure whose western flank is bounded by the KES (Figure 3). The eastern flank of the massif is delimited by the steeply east dipping, right-slip Kuke fault, which appears to continue northward to become the Gez fault [Robinson et al., 2004, 2007]. Previous work suggests that the Kuke fault forms the westernmost strand of the KYTS [Cowgill, 2010], which experienced a significant dip-slip component, with west-side-up displacement between ~12 and 6 Ma (40Ar/39Ar biotite and ZHe ages) [Sobel et al., 2011]. The core of the Muztagh Ata dome experienced prograde metamorphism (9–10 kbar, 700°C–750°C) at 30–14 Ma (monazite Th-Pb inclusion ages in garnet) and cooled through the biotite closure temperature by ~8 Ma [Robinson et al., 2007]. Along the southern termination of the Muztagh Ata dome, top-to-the-south directed normal faulting has been documented, which may be related to the Shen-ti fault, accommodating north-south directed domal exhumation prior to the onset of east-west oriented normal faulting. This interpretation is supported by a sharp break in biotite and muscovite 40Ar/39Ar cooling ages, a significant change in metamorphic grade, and top-to-the-south kinematic indicators [Robinson et al., 2007]. Based on these observations, Robinson et al. [2007] suggested that the Muztagh Ata basement rocks form the eastern continuation of the Shatput dome. In their view, the Shen-ti fault constitutes a southeast dome-bounding detachment fault, although cooling determined for this area is >5 Myr younger than cooling ages obtained from the dome farther west [Schwab et al., 2004]. Within the core and southern termination of the Muztagh Ata massif, the orientation of the foliation rotates from west dipping to subhorizontal (see Figure 4b) and the topography is characterized by moderately high relief (1–2 km; see Figure 3b). The Kongur Shan and Tahman normal faults clearly cross-cut the southern flank of this antiform, and therefore the domal structure of the Muztagh Ata massif is interpreted to largely predate the initiation of east-west oriented normal faulting along the KES. This is notably different from the structure of the Kongur Shan massif to the north, where dome formation is interpreted to be synchronous with east-west extension. In summary, previous studies have well documented the timing of prograde peak metamorphism as well as the onset of exhumation of the eastern Pamir gneiss domes. However, these studies reveal very little about later stages of deformation, the duration, and lateral variation of exhumation of the domes and the KES system.

Figure 4.

(a) Field photograph taken from the northern slope of the entrance to Gez River gorge looking southeast at the age-elevation transect in the footwall of the Kongur Shan normal fault. Black arrows point to a warped normal-faulted detachment surface. White squares show sample locations with elevation and measured ZHe, 40Ar/39Ar biotite (Ar-bt), and white mica (Ar-ms) cooling ages in million years. (b) Field photograph looking from the core of Muztagh Ata north down toward the Kuksay valley. Note that within the core of the Muztagh Ata massif, the foliation rotates from west dipping to subhorizontal.

3 Thermochronologic Data

[18] In order to address the most recent tectonic evolution of the northeast Pamir during the neotectonic extensional regime and its relationship with earlier deformation, we present new AFT, ZHe low-temperature thermochronometry data, and new 40Ar/39Ar mica ages (Table 1). We combined this data set with previously published data, predominantly 40Ar/39Ar mica ages, to determine the spatial and temporal characteristics of cooling. We applied one-dimensional thermal modeling to this data set from both hanging-wall and footwall rocks of the KES to derive exhumation rates. Analytical procedures for the AFT, ZHe, and 40Ar/39Ar step-heating techniques are documented in the data repository and are the same as those described in the work of Sobel et al. [2011, and references therein].

Table 1. Results of new AFT, ZHe and 40Ar/39Ar mica analysis
Sample IDLatLongAltLithoMethLocationAge P(χ2)%GeothØt-ExhComment
(DD)(DD)(m)(Ma)(±σ)or FT(°C/km)(mm/yr)(±σ)
  1. Analytical procedures for the AFT, ZHe and 40Ar/39Ar step-heating analyses techniques are the same as those described in Sobel et al. [2011] and references therein. Analytical details of the measurements are provided in supplementary material in Tables S1 and S2. Abbreviations: Lat (DD) – latitude in digital degree; Long (DD) – longitude in digital degree; Alt – altitude; Litho – Lithology; Meth – dating method; Location – sample location; P(χ2)% - percentage of chi square value; estimated Geoth – geothermal gradient; Øt-Exh – time averaged exhumation rate; metaSS - metamorphosed-sandstone; metavolc - metamorphosed volcanic rock; hw - hanging wall of KES; fw - footwall of KES.
P05T-1338.9215774.746423720graniteAFTKATT - hw detachment5.30.620201.00.4 
ZHe12.41.00.761.00.3Ø of 3 aliquots
P05T-1438.9164274.745483630graniteAFTKATT - hw detachment4.40.445201.10.4 
P05T-1738.9259174.786753980metaSSAFTKATT - fw detachment3.40.517201.60.6 
ZHe3.60.30.782.50.9Ø of 3 aliquots
P05T-2138.9208274.780804230metaSSAFTKATT - fw detachment2.90.755202.00.8 
P05-T2338.9153474.784923810metaSSAFTKATT - fw2.50.56202.10.9 
P05T-3138.2932775.034184640metavolc?AFTMuztagh Ata -fw1.70.368501.10.4 
P05T-3238.2885475.050615380metavolc?AFTMuztagh Ata -fw3.90.744500.50.1 
P05T-3738.0940275.256953765gneissZHeTahman-fw6.30.51500.50.1Ø of 3 aliquots
P05-T3938.0816475.236473580gneissAFTTahman-fw5.40.849500.40.1 
AFTTahman-fw6.11.0230.40.1 
P05T-4138.0762775.230453510gneissAFTTahman-fw5.00.678500.40.1Ø of 3 aliquots
ZHeTahman-fw5.20.40.790.60.1
P05T-6338.3746175.212273987metaSSAFTMustagh Ata -fw1.70.2695010.4 
ZHe1.10.10.822.30.7Ø of 3 aliquots
P05T-6438.4366275.038103660metaSSZHeKarakol lake -hw6.80.50.75500.50.1Ø of 3 aliquots
P05T-6538.5603675.008063380gneissAFT 5.90.989500.90.3 
AFTN of Karakol5.80.6590.90.3 
ZHe 9.70.84.400.40.1Ø of 3 aliquots
P05T-7038.7107175.065944862graniteZHeKongur Shan -fw0.70.17.97503.21.3Ø of 3 aliquots
Ar bt1.60.1    
P05T-7238.7158675.060994375graniteAFTKongur Shan -fw1.60.57750  "too old"
ZHe0.60.16.633.41.4Ø of 3 aliquots
P05T-7338.714675.065664580graniteZHeKongur Shan -fw1.20.14.75502.20.6Ø of 3 aliquots
Ar bt1.90.0 2.10.2 
P05T-7538.7226475.046563600graniteAFTKongur Shan -fw2.30.64350  "too old"
ZHe0.50.00.793.71.5Ø of 3 aliquots
Ar bt1.30.1 2.80.3 
Ar mu2.50.1 2.00.3 
PO5-T7738.7711975.197172790graniteAFTKongur Shan -fw1.10.24050  "too old"
 ZHe1.00.10.752.30.3Ø of 3 aliquots
P09T-2338.8697374.526443720graniteZHeKATT - fw detachment14.51.20.81200.90.2Ø of 3 aliquots
P09T-2538.3477875.28415???graniteAr btKanxiwar5.01.0 50--no bedrock
P09T-2438.4160375.179793690f. augen gneissZHeKanxiwar4.10.30.75500.90.2Ø of 3 aliquots
P09T-2738.2410275.368064660felsic gneissZHeKanxiwar4.10.30.77500.90.2Ø of 3 aliquots
P09T-2838.2636775.338044400gneissZHeKanxiwar3.20.30.76501.00.2Ø of 3 aliquots
P09T-3038.3734475.258533800graniteZHeKanxiwar2.50.20.81501.20.3Ø of 3 aliquots
P05T5837.8764075.286693020felsic gneissZHeTashkorgan River5.60.50.78500.60.1Sobel et al. 2011
P09T-137.8700975.351833000felsic gneissZHeTashkorgan River6.50.50.80500.50.1Sobel et al. 2011

[19] To measure along-strike variation in fault displacement along several segments of the KES, a series of 16 AFT and 18 ZHe samples were analyzed, yielding cooling ages between 6.1 and 1.1 Ma and 6.3 and 0.5 Ma, respectively (Figures 3-5 and Table 1). Spot hanging-wall samples yield older AFT as well as ZHe cooling ages, ranging from 5.9 to 4.4 Ma and 14.5 to 6.8 Ma, respectively. In addition, we obtained ZHe and 40Ar/39Ar mica ages from samples collected along an elevation transect parallel to the Kongur Shan normal fault at the entrance of the Gez River gorge (see Figure 4). Our new data are presented together with published data [Robinson et al., 2004, 2007; Robinson et al., 2010; Sobel et al., 2013]. Significant lateral variations in the footwall cooling ages document deep-seated, along-strike variations in exhumation and fault displacement during the late Miocene-Pliocene.

Figure 5.

Topographic swath profiles oriented perpendicular to the strike of different fault segments of the (a–d) KES and (e) Shen-ti fault. See inset and Figure 2 for locations. New and compiled 40Ar/39Ar mica, ZHe, and AFT cooling ages plotted by their relative distance (kilometers) to main fault trace versus cooling age (Ma, in logarithmic scale). Topographic swath profiles are based on SRTM (90 m)-derived DEM, with mean elevation shown as a black line, 2σ standard deviation of mean elevation as a dark gray envelope, and maximum and minimum topography as light gray envelopes. The relative location of the fault plane is shown as a dashed black line with arrows indicating the direction of fault movement. TBF is Torbashi fault.

4 Fault Segments Along the KES

[20] We have plotted and analyzed our data along four transects oriented perpendicular to the regional strike of the four KES fault segments from north to south (Figures 5a–5d). A fifth transect is oriented north-south, perpendicular to the strike of the Shen-ti fault (Figure 5e).

4.1 King Ata Tagh Transect

[21] The King Ata Tagh transect (KATT) is representative of the northwestern segment of the KES (Figures 3a and 5a). Along this segment, the exposure of the hanging-wall rocks is good, because of thin to absent basin fill. The main fault plane is well exposed; lineations and fault kinematic indicators measured in the footwall suggest top-to-the-south normal faulting with an oblique dextral component. KES hanging-wall samples yield mean ages of 4.8 ± 0.5 Ma (AFT, n = 2) and 13.5 ± 1.1 Ma (ZHe, n = 2); footwall samples yield mean ages of 2.9 ± 0.5 Ma (AFT, n = 3) and 3.6 ± 0.3 Ma (ZHe, n = 1). Previously published 40Ar/39Ar mica ages range from ~137 to 167 Ma in the hanging wall and ~80 to 100 Ma in the footwall. In addition, a previously published 40Ar/39Ar K-feldspar age combined with multidiffusion domain modeling suggests slow cooling or isothermal conditions throughout the Cenozoic until ~7 Ma and increased cooling at a constant rate since then. This indicates that cooling through the Ar closure temperature occurred prior to the Cenozoic, fault displacement along the KES initiated along this segment at ~7 Ma, and overall the amount of Cenozoic fault displacement has been less compared to the gneiss domes to the south [Robinson et al., 2004; Sobel et al., 2013].

4.2 Gez River Transect

[22] The Gez River transect (GRT) was sampled along the southwestern flank of the Kongur Shan massif. The transect follows the Gez River gorge and runs from immediately below the fault surface to approximately 15 km into the footwall. The analysis yields mean ages of 0.8 ± 0.1 Ma (ZHe, n = 5) and 2.3 ± 0.6 Ma (AFT, n = 1) between the KES and the Gez Fault and 1.4 ± 0.1 (ZHe, n = 2) and 1.1 ± 0.2 (AFT, n = 1) to the east of the Gez fault. Previously published 40Ar/39Ar mica ages yielded mean ages of ~2 Ma near the KES, increasing to the east to ~5 Ma, just west of the Gez fault [Robinson et al., 2004; Robinson et al., 2010; Sobel et al., 2013].

[23] We also collected an elevation transect along the footwall of the Kongur Shan normal fault between ~3600 and 4900 m asl, parallel to the fault plane along the western dome flank (see Figures 3 and 4). The transect yields consistently young cooling ages of <1 Ma ZHe, 40Ar/39Ar biotite ages of <2 Ma, and a 40Ar/39Ar muscovite age of 2.5 Ma, in good agreement with Robinson et al. [2004, 2010]. The AFT ages are older than the ZHe ages and 40Ar/39Ar mica ages, which is inconsistent with the closure temperature model. Metamorphic growth of the apatites resulting in low and inhomogeneous distribution of U during very rapid cooling is the most likely cause for this inconsistency in our analysis. Most probably for the same reasons, our attempts to perform apatite (U-Th)/He analysis on these samples were unsuccessful. The AFT data for these samples are therefore excluded from further analysis.

4.3 Northern Muztagh Ata Transect

[24] The Northern Muztagh Ata transect (NMT), a 10 km long transect from the gneisses of the footwall into the core of the Muztagh Ata massif along the Kuksay glacial valley, yields a footwall ZHe age of 1.1 ± 0.1 Ma close to the fault of the KES, a mean age of 3.7 ± 0.3 Ma (n = 2) in the dome interior, and two AFT ages of 1.7 ± 0.2 and 3.9 ± 0.6 Ma along the footwall farther west. Similar to the AFT results obtained along the GRT, AFT ages are interpreted to be of low quality and therefore disregarded for further analysis. Two 40Ar/39Ar mica ages of 6.2 and 2.7 Ma show a weak trend toward older ages to the south along the trace of the fault, although the data are limited in number [Robinson et al., 2007].

4.4 Southern Muztagh Ata Transect

[25] The Southern Muztagh Ata transect (SMT) follows the main canyon at the southern termination of the west-plunging culmination of the Muztagh Ata dome. The canyon runs ~7 km into the footwall of the dome and yields mean ZHe ages of 5.8 ± 0.5 Ma (n = 2) and AFT ages of 5.6 ± 0.8 Ma (n = 2). Previously published 40Ar/39Ar biotite ages range between 7.5 and 8.5 Ma [Robinson et al., 2007].

4.5 Shen-ti Fault Transect

[26] The Shen-ti fault transect (SFT), a north-south trending, 30 km long transect, extends from the hanging wall of the Shen-ti fault into the Muztagh Ata massif. Recently published ZHe ages from the footwall of the Shen-ti Fault are 5.6 ± 0.5 Ma and 6.5 ± 0.5 Ma in the hanging wall [Sobel et al., 2011]. Monazite Th-Pb inclusion and matrix ages in garnet located in the footwall yield peak metamorphic ages of 30–14 Ma (n = 22) and post-peak metamorphic ages of 25–8 Ma (n = 63), respectively [Robinson et al., 2007]. 40Ar/39Ar biotite ages are consistently between 7.5 and 9.0 Ma (n = 22). The hanging wall of the Torbashi fault yields ages of >100 Ma (n = 3); this block represents the hanging wall of the Shen-ti fault. This suggests that cooling in the footwall of the Shen-ti fault was rapid prior to ~8 Ma but that fault displacement ceased afterward, although not later than ~5 Ma.

5 Interpretations From Thermal Modeling

5.1 Along-Strike Variation in Geothermal Gradients and Exhumation Rate

[27] The estimation of the closure temperature depth from thermochronometric data may be achieved using one-dimensional, two-dimensional, or three-dimensional thermal models of varying complexity. As to which modeling approach is most favorable depends on the availability of external constraints, such as topographic evolution, and structural and thermal information about the deformed upper crust [Braun, 2002; Ehlers, 2005; Reiners and Brandon, 2006]. During pervasive crustal deformation, such as the emplacement of basement domes, where hot footwall rocks from great depths are rapidly uplifted and exhumed, the geothermal field is strongly advected and isotherms are compressed, resulting in very steep geothermal gradients [e.g., Lee et al., 2000; Teyssier and Whitney, 2002; Beaumont et al., 2004; Lee et al., 2004, and references therein]. Previous thermal modeling of the Kongur Shan massif demonstrates this advection, illustrating the challenges involved in accurately estimating exhumation when rates are on the order of several mm a−1 [Robinson et al., 2010]. Therefore, we have concentrated on using one-dimensional models and focused on relative, rather than absolute, variations in exhumation along the strike of the KES. Methodological studies have demonstrated that one-dimensional models are often sufficient to provide first-order constraints needed to transform cooling ages into exhumation rates, as the vertical component of rock deformation is the most important [Whipp et al., 2007, 2009].

[28] The low-temperature thermochronologic cooling ages presented in this study have been inverted to obtain time-averaged denudation rates using one-dimensional AGE2EDOT models [Brandon et al., 1998] (latest version published in Ehlers et al., [2005]). These forward models of the thermal response to exhumation were applied using a one-dimensional finite difference approximation to the advection-diffusion equation. A range of exhumation histories were tested, each subjected to the variations of the initial geothermal gradient between 20°C km−1 and 50°C km−1, thermal conductivity of the crystalline basement between 2 and 3.7 Wm−1 K−1, and surface radiogenic heat production between 0.8 and 2.5 μW m−3, with an annual mean-surface temperature of 5°C. An evaluation of how well these thermal histories match the thermochronologic data was accomplished by comparing predicted and measured cooling ages. Because the kinetics of diffusive loss of He from zircon are not completely understood, we simply track the 180°C isotherm as a proxy for the closure temperature of this system. All cooling ages were plotted along strike of the KES (Figure 6a) and converted into exhumation rates to determine relative, rather than absolute, variations of exhumation (Figure 6b).

Figure 6.

(a, c) Lateral variation in cooling ages (Figure 6a) and one-dimensional thermal modeling-derived exhumation rates (Figure 6c) are plotted along strike of the KES. Significant variation in geothermal gradient along strike is assumed based on geological constraints as explained in the text. The KES is subdivided into four different segments characterized by temporal and spatial variations in faulting from north to south, plus one segment along the Shen-ti fault. (b) Four 15 km wide swath profiles (S1–S4) running parallel to the ridge line of the domal massifs have been calculated (see Figure 3b for location). The bold black line represents the mean elevation, with an envelope in gray showing the maximum and minimum elevations. The bold red line represents the mean relief, with an envelope showing maximum and minimum relief. Topographic relief was calculated using a 5 km moving circle across the SRTM (90 m)-derived digital elevation data set. KATT-KingAta Tagh transect, GRT-Gez River Transect, NMT-north Muztagh Ata transect, SMT-south Muztagh Ata transect,770 and SFT-Shen-ti fault transect.

[29] We conducted sensitivity tests (see supporting information), which demonstrated that an interpretation of the thermal history in this kind of setting depends more on initial parameters such as geothermal gradient rather than thermophysical rock parameters such as thermal diffusivity and internal heat production. Thermal modeling by Robinson et al. [2010] in the vicinity of the rapidly incising Gez River through the Kongur Shan massif yielded geothermal gradients up to ~100°C km−1 after onset of extension and doming. However, because the input for this model is spatially clustered and due to the combination of rapid river incision, which creates a deep gorge during rapid rock uplift and exhumation of the dome, we expect that elevated isotherms are more tightly compressed, resulting in higher cooling rates and steeper geothermal gradients at this location. It is therefore difficult to assess whether this is a representative value for the entire Kongur Shan massif. In our study, we have favored a more conservative estimate of ~50°C km−1 for the regional mean steady-state geothermal gradient during exhumation of KES footwall rocks. Recent rapid exhumation of the Kongur Shan massif, where temporal constraints suggest a short lag of <10 Myr between peak metamorphism (9 Ma, Th-Pb monazite inclusion in garnet) and exhumation of these rocks to the surface, provides an analog for the thermal effects of other Pamir domes in similar settings. On this basis, we assume strong disturbances of the regional geothermal gradients within the crust in the vicinity of Muztagh Ata dome when it was exhumed during the middle to earliest late Miocene [Robinson et al., 2007], as well as in the vicinity of the Muskol, Shatput, and Sarez gneiss domes, during uplift and exhumation in central Pamir during the late Oligocene-early Miocene times [Schmidt et al., 2011]. Prograde metamorphism between 30 and 14 Ma, constrained by Th-Pb Monazite geochronology, and the cessation of Ar loss for biotite and white mica by ~8 Ma suggest that major portions of the Muztagh Ata dome were exhumed prior to the evolution of the KES and with rates similar to those obtained along the Kongur Shan. Therefore, a geothermal gradient of at least ~50°C km−1 prior to ~8 Ma appears to be realistic for this area, too. Only for the fault segments located northwest of the Kongur Shan, where published 40Ar/39Ar ages of >100 Ma indicate the absence of Cenozoic deep-seated exhumation [Robinson et al., 2004], can a normal geothermal gradient of ~20°C km−1 be assumed prior to the onset of exhumation. This assumption is supported by regionally distributed ZHe (25–20 Ma) and AHe (20–15 Ma) cooling ages (Figure 7) obtained from samples collected along eastern ranges bounding the extensional Lake Karakul basin as well as adjacent to the Muji basin, suggesting very low erosion and exhumation since the middle Miocene [Amidon and Hynek, 2010; Sobel et al., 2013].

Figure 7.

Regional compilation of new and previously published 40Ar/39Ar mica, ZHe, AFT, and AHe cooling ages [Arnaud et al., 1993; Robinson et al., 2004; Schwab et al., 2004; Robinson et al., 2007; Robinson et al., 2010; Sobel et al., 2011; Sobel et al., 2013, and this study] and their relative position to the first-order tectonic lineaments throughout the northeast Pamir [M.R. Strecker et al., 1995; Robinson et al., 2004, 2007]. Note the lateral variation in cooling ages along the different segments of the KES, strong variation in cooling ages obtained in the vicinity of the normal fault systems of Lake Karakul in the north-central Pamir, and preliminary cooling data in the footwall of early Miocene east-trending domes of the central Pamir. White rectangles show the location of the topographic swath profiles in Figure 5.

[30] From these observations and the sensitivity tests, we derive three general conclusions regarding the estimation of exhumation by one-dimensional thermal modeling. (1) Minor variations in exhumation rates resulting from varying thermal diffusivity and heat production properties of the rocks indicate that the natural variability of thermophysical properties translates to minor and negligible variations in cooling ages in this setting; therefore, our assumptions are robust. (2) Modeling results indicate that constant exhumation under the unusually high geothermal gradient of ~50°C km−1 results in significantly younger cooling ages, translating to roughly half as high exhumation rates, compared to the settings with normal geothermal gradients (~20°C km−1). (3) As previous studies have demonstrated in other areas [Ehlers, 2005; Reiners and Brandon, 2006], when exhumation rates are high (AFT >1 mm a−1, ZHe >1.5 mm a−1, Ar-bt >2 mm a−1, Ar-ms >2 mm a−1), the resulting highly advected and compressed isotherms cause precision of inverted exhumation rates to drop by more than twofold.

[31] Taken together, our new thermochronologic results and modeling suggest strong along-strike temporal and spatial variations in footwall exhumation of the KES. In particular, our data illustrate that the growth of the domal massifs has been successive and exhumation occurred in two separate events, despite the lack of a clear tectonic boundary. Furthermore, the formation of the Muztagh Ata dome predates initiation of east-west extension along the KES. We accordingly divide the discussion of the KES fault data into two sections. The first section focuses on the KES fault running along the base of the Kongur Shan massif (GRT in Figure 6, see red arrow) and fault segments to the northwest (KATT in Figure 6). The second section addresses the KES fault running along Muztagh Ata and exhumation of the core of the dome (NMT in Figure 6), as well as the area at the southwestern termination of the massif (SMT and SFT in Figure 6).

5.2 Kongur Shan Basement Dome (GRT) and Northwestern KES Fault Segments (KATT)

[32] Our results are consistent with earlier findings that significant exhumation has been focused on the vicinity of the Kongur Shan massif since the late Miocene [Robinson et al., 2004, 2007]. The >4 km high topographic relief (Figure 3b), young and elevation-invariant 40Ar/39Ar mica ages of 1.5–2.5 Ma, and ZHe of ~1 Ma obtained along the elevation transect parallel to the Kongur Shan normal fault confirm rapid cooling during Pliocene exhumation, which continues to the present day. Recent thermal modeling results presented by Robinson et al. [2010] demonstrate that the rapid exhumation observed today is best explained by protracted and continuous high rates (~4 ± 1 mm a−1), compressing isotherms since the late Miocene. These authors concluded that this is related to changes in isothermal heat advection rather than to increases in exhumation rate during the Quaternary as previously suggested [Arnaud et al., 1993]. For comparison, and in good agreement with these results, our one-dimensional thermal modeling approach yields vertical footwall exhumation rates of >3–2.5 ± 1.0 mm a−1. A component of cooling along the KES, particularly along the Kongur vertical profile, is due to tectonic denudation. However, given these rapid footwall exhumation rates and >4 km of topographic relief (Figure 3b), a significant portion of the Pliocene exhumation must be enhanced due to river and/or glacial incision and erosion.

[33] Along the KATT (Figure 6), fault displacement can be inferred due to differences in ZHe and AFT cooling ages in the hanging wall and footwall. Footwall ZHe and AFT cooling ages between 2.5 ± 0.5 and 3.6 ± 0.3 Ma are interpreted in terms of ongoing cooling during faulting along KES fault segments located northwest of Kongur Shan. Assuming a normal geothermal gradient of ~20°C km−1 prior to faulting, these cooling ages suggest rapid vertical exhumation of the KATT footwall segment with rates of ~2 ± 0.5 mm a−1. As previously published 40Ar/39Ar mica ages are not reset by faulting and 40Ar/39Ar feldspar analysis combined with multidomain diffusion modeling suggests that faulting started by ~7 Ma [Robinson et al., 2004], exhumation is not as deep-seated as along the Kongur Shan gneiss dome. By inverting hanging-wall AFT and ZHe cooling ages of 4.4 ± 0.4 to 5.3 ± 0.6 Ma and 12.4 ± 1.0 to 14.5 ± 1.2 Ma, respectively, an exhumation rate of <1 mm a−1 is obtained.

5.3 Muztagh Ata Basement Dome

[34] The main phase of exhumation of the Muztagh Ata basement dome occurred between ~14 and 8 Ma [Robinson et al., 2007]. Repeated intrusion of igneous melts into shallow crustal levels of the Muztagh Ata hanging wall between 18 and 11.5 Ma [Xu et al., 1996; Zhang et al., 1996; Robinson et al., 2007; Jiang et al., 2012] suggests that there has been additional injection of heat into the upper crust in the vicinity of the southwestern margin of the dome. The coeval footwall and hanging-wall ages and similar exhumation rates (see SMT (d) and SFT (e) in Figure 6) indicate that the entire region cooled coevally through the respective closure temperatures, indicating either regional cooling following magmatism or rapid exhumation, as suggested by Robinson et al. [2007]. If early faults related to the KES had already been active, their displacements were insensitive to this regional cooling.

[35] The four new ZHe ages of 3.2 ± 0.3 to 4.1 ± 0.3 Ma from the core of the Muztagh Ata dome, located ~10 km southeast of the northern shear zone, indicate moderate but continuous exhumation of ~1 ± 0.5 mm a−1 from the Pliocene to the present day. This observation is consistent with recently published thermochronologic results along the east-west oriented Tashkorgan River gorge transect along the southern termination of the dome [Sobel et al., 2011], indicating that the core of this basement dome is still being deformed. Interestingly, the western tip of the Muztagh Ata culmination where the KES bends around is characterized by unusually high topographic relief (~4 km; Figure 3b) compared with the central and southern segments of the dome (only 1–2 km relief on average) and suggests that this portion of the dome has exhumed during faulting along the KES most recently. (Topographic relief was calculated using a 5 km moving circle across the SRTM 90 m digital data set.) New AFT and ZHe cooling ages of 5 to 6 Ma in the vicinity of the southwestern flank of the Muztagh Ata dome (e.g., shown in transects d and e in Figure 6) suggest significantly less fault displacement along the KES toward the south during the late Mio-Pliocene. Mean footwall exhumation rates slowed down to 0.5 to 0.4 ± 0.1 mm a−1 for the last 5 Ma and are in the same range as the hanging-wall exhumation rates. This and only moderately high topographic relief (1–2 km; Figure 3b) suggest limited fault displacement in the south, confirming the results of earlier work [Robinson et al., 2007]. Early faulting (prior to 9 Ma) along the southern termination of the Muztagh Ata Dome was related to top-to-the-south directed normal faulting, such as along the Shen-ti fault [Robinson et al., 2007]. Therefore, we interpret AFT and ZHe cooling ages to reflect the post-tectonic thermal relaxation of advected and compressed isotherms, rather than as cooling related to faulting along the KES.

[36] In summary, the combination of our new and previously published thermochronologic data and the characteristics of the present-day topographic relief (see Figures 3a and 3b) suggest that relatively little fault displacement occurred along the KES in the vicinity of the southwestern flanks of the Muztagh Ata dome. However, moderate exhumation in the doubly plunging antiformal core suggests ongoing crustal warping.

6 Discussion

6.1 Evolution of the KES

[37] Our data provide new insights into along-strike variations in fault displacement of the KES, which we interpret to reflect a late Cenozoic kinematic change in deformation and tectonic evolution within the upper crust in the northeast Pamir. We first discuss the local geological implications and subsequently address the late Cenozoic evolution of the northeastern Pamir-Tien Shan convergence zone in the context of recently published geodetically derived crustal velocity deformation and tectonic scenarios. Finally, we present a new model suggesting how the observed deformation patterns could be interpreted in the context of the Pamir indentation.

[38] The along-strike variation in topographic relief (Figures 3b and 6) and crustal cooling (see Figures 3-7) reveals significant spatial disparity in the magnitude and timing of fault-related exhumation (Figure 6) and requires a more complex faulting history than previously envisioned. The trace of the KES is morphologically well expressed for at least 80 km to the northwest and south of both domal massifs; however, only in the vicinity of the domes do mica 40Ar/39Ar cooling ages record Mio-Pliocene exhumation [Robinson et al., 2004, 2007]. Combining new and previously published data reveals that deep-seated exhumation occurred successively along the fault segments bounding both domal massifs, first along Muztagh Ata in the south and later along Kongur Shan in the north. Earlier findings suggested that the Muztagh Ata basement dome underwent major exhumation between 14 and 8 Ma [Robinson et al., 2007], whereas the rocks of the Kongur Shan have been exhumed from 8–7 Ma until the present [Robinson et al., 2004]. Along the southern Muztagh Ata and Tahman Fault segments, the main pulse of exhumation occurred prior to 8–7 Ma, before the onset of E-W extension. Partly overlapping 5 to 6 myr old AFT and ZHe cooling ages (transects d and e in Figure 6) support the notion of rapid cooling prior to 8 Ma, which subsequently slowed down, indicating minor fault displacement during E-W normal faulting along the Muztagh Ata massif and areas farther south. This is compatible with recently published ~6 myr old ZHe cooling ages in the footwall of the Kuke fault that suggest cooling prior to E-W extension and decreased exhumation since then [Sobel et al., 2011]. However, 3–4 Ma ZHe cooling ages from rocks exposed in the core of the Muztagh Ata massif (this study) as well as along the east-west trending Tashkorgan River gorge [Sobel et al., 2011] suggest continuous slow warping and exhumation (<~1 mm yr−1) in the center of the dome.

[39] The growth of many of the Pamir domes occurred during an early stage of the tectonic evolution of this mountain belt, approximately 20–30 Ma after the onset of the India-Eurasian collision. The unifying characteristic of these domes in the western and central Pamir, including the Shakhdara, Yazgulom, Sarez, Muskol, and Shatput domes, is the E-W orientation of their long axes, except in the western sector of the Pamir salient where the axes have been bent into a NE-SW orientation as a result of the indentation processes (see Figure 2). In contrast to these domes, the Muztagh Ata and Kongur Shan domes grew parallel to the India-Eurasia convergence direction. Furthermore, intermediate-depth seismicity indicates that the subducted slab strikes E-W below the Muztagh Ata massif [Sippl et al., 2013]. The relatively sharp eastern termination of the subducted slab east of the culmination of the Muztagh Ata massif and its correlation with the surface expression of the KYTS led Sobel et al. [2013] to hypothesize that a crustal tear runs parallel to the KYTS in the subsurface. This tear is interpreted to be a consequence of the lateral termination of the subducting Alai slab, which allowed the Pamir to move north with respect to the Tarim Basin and may have provided favorable conditions for mantle heat transfer into the upper crust. This feature could explain the location and composition of igneous intrusions derived from the lithospheric mantle and emplaced immediately to the west of the Muztagh Ata massif (see Figure 7 for location) [Xu et al., 1996; Ke et al., 2006; Robinson et al., 2007; Jiang et al., 2012]. The overlap in age between the zircon U-Pb analysis (11.7 ± 0.5 Ma) and 40Ar/39Ar analyses on biotite (11.3 ± 0.3 Ma) and K-feldspar (11.6 ± 0.8 Ma) suggests that the intrusions were emplaced at shallow crustal depths and cooled rapidly during growth of the Muztagh Ata massif. Crustal xenoliths with Asian affinity in volcanic host rocks were derived virtually contemporaneously from mantle depths of ~90 km and cooled rapidly in the southeastern Pamir, as documented by biotite 40Ar/39Ar ages of 11.0 to 11.5 ± 0.3 Ma [Ducea et al., 2003; Hacker et al., 2005; Gordon et al., 2012]. Regardless of whether these xenoliths are the result of lower crustal foundering or subduction erosion [Hacker et al., 2005] or are related to a crustal tear in the subducting Asian slab [Sobel et al., 2013], their existence suggests that the East Pamir crust may have been injected by hot ~1000°C to 1100°C melts [Gordon et al., 2012]. If the volume of these intrusions had been large enough, they would have caused significant heating of middle and upper crustal levels, possibly generating crustal anatexis and decreasing crustal strength, compared to the central and west Pamir. In addition, such melt generation might have fostered the exhumation of the Muztagh Ata and Kongur Shan domes by increasing their buoyancy [Teyssier and Whitney, 2002], thus permitting crustal weakening and extension.

[40] Ultimately, the crustal tear in the subducting slab beneath the East Pamir would have caused greater heating along the eastern margin, allowing for localization of deformation and decoupling between the central/western Pamir and eastern Pamir/Tarim basin. Clearly, more data are needed to test this hypothesis, but the remarkably similar spatial and temporal evolution of these features suggests an enticing new causal mechanism to explain the late Cenozoic tectonic evolution of the eastern Pamir.

6.2 Models of the Origin of East-West Extension

[41] Several models have been proposed to explain the origin of east-west extension in the northeastern Pamir; these make differing predictions about variations in the spatial distribution and timing of extension. Brunel et al. [1994] suggested that the KES is the result of synorogenic extension, during motion along the MPT, associated with crustal-scale folding and normal faulting. This model predicts both the initiation and largest magnitude of extension to be along the domes in the center of the extensional system, with the initiation age and magnitude of extension decreasing to the northern and southern tips of the fault systems. This agrees well with our new constraints. However, along the eastern margin of the Pamir, the MPT is not a pronounced tectonic feature. Rather, deformation there appears to have been dominated by movement along the transpressional KYTS [Cowgill, 2010]. Indeed, shortening along the MPT is highly oblique to slip along the KES, making kinematic coupling between the two systems problematic. Also, an increasing body of evidence demonstrates that the domes formed in two separate events, rather than forming simultaneously, as it would be predicted by this model. In addition, the onset of dome formation postdates the initiation, although poorly constrained, of slip along the MPT [Sobel and Dumitru, 1997] and slip along the KES, most likely by >10 Myr [Robinson et al., 2004, 2007]. Therefore, this model provides a good explanation for neither the initiation of the KES nor east-west crustal extension.

[42] In a subsequent model, Strecker et al. [1995] proposed that extension in the Pamir may be driven by radial thrusting of the Pamir salient along the MPT. This model predicts initiation of extension along the northern portion of the fault, propagating southward, with the greatest magnitude of extension along the northern portion of the fault. However, more recent structural observations have documented strike-slip faulting rather than thrusting to be the dominant mode of regional faulting [Cowgill, 2010] and fault displacement rates ceased along the eastern margin of the Pamir during the late Miocene [Sobel et al., 2011], when normal faulting along the KES initiated, making this model untenable.

[43] In a related alternative model, Yin et al. [2001] proposed that extension may be driven by oroclinal bending of the entire Pamir and western Himalaya region. This model also predicts fault propagation from north to south, with the greatest magnitude of extension in the north. However, there is no evidence for this pattern across the entire Pamir as normal faulting is limited to the north-central and east Pamir. In addition, existing paleomagnetic data do not support significant clockwise rotation of the eastern Pamir [Yin et al., 2001].

[44] It has also been proposed that extension within the northern Pamir is kinematically linked to the dextral Karakorum fault [Ratschbacher et al., 1994; Murphy et al., 2000; Chevalier et al., 2011]. This model considers east-west extension in the Pamir as part of the overall extension across Tibet and specifically links the KES with extensional structures in southwestern Tibet via the right-slip Karakorum fault (Figure 4c). According to this model, transpressional deformation in the Pamir resulted from indentation of the western syntax of India into Asia, combined with overall anticlockwise rotation of India during its northward migration [e.g., Patriat and Achache, 1984; DeMets et al., 1990; Le Pichon et al., 1992]. The recently proposed 30%–40% slowdown of India-Asia convergence (northwest corner of India) from 4.4 to 3.4 cm/yr between 20 and 10 ±1 Ma [Merkouriev and DeMets, 2006; Molnar and Stock, 2009] was accompanied by a 20° counterclockwise rotation of the convergence direction [DeMets et al., 1990; see Le Pichon et al., 1992]. Whether such reorientation of the convergence direction coincides with and accounts for the onset of NE-SW and E-W extension across the KES and exhumation of the Kongur Shan-Muztagh Ata gneiss domes remains speculative. Furthermore, recently published alternative plate reconstructions [Copley et al., 2010] using slightly varying rotation parameters in the Atlantic and Indian Oceans do not show the significant velocity change at ~20–10 Ma as described above [Le Pichon et al., 1992; Merkouriev and DeMets, 2006; Molnar and Stock, 2009] and illustrate the difficulty of unambiguously reconstructing plate motions. In addition, this model predicts initiation of the extension system in the south, with northward propagation and with the greatest magnitude of extension along the southern portion of the fault system. However, Robinson et al. [2004, 2007, 2010] demonstrated that the magnitude of extension is greatest in the vicinity of the Kongur Shan dome and decreases southward. This is in excellent agreement with only a moderately high (1–2 km) topographic relief (see Figures 3 and 6 for comparison) and our own new AFT and Z-He data, which also suggest decreased fault displacement along the KES along the southern portions of the Muztagh Ata massif. This is also compatible with satellite image-derived Quaternary fault constraints farther south [Robinson et al., 2004]. In addition, Robinson et al. [2007] and Robinson [2009] have proposed that the northern portions of the Karakorum fault have been inactive recently and that the KFS and KES are not kinematically linked. All these reasons suggest that the KES extension pattern is not driven by the same processes that are controlling extension in west and south-central Tibet.

[45] Finally, crustal thickening immediately prior to east-west extension along the KES and the apparent decrease in magnitude of east-west extension both to the north and to the south provides evidence to support synorogenic extension in which extension develops as a result of locally thickened crust by vertical wedge extrusion [Robinson et al., 2004].

6.3 New Scenarios for East-West Extension of the Northeastern Pamir

[46] In light of our new data, we propose an alternative model. We first review recent space geodetic studies and put these in the context of the variations in exhumation rate along the KES and its role in regional tectonics.

[47] GPS-derived crustal velocity vectors (see Figure 2) indicate that today the northeast margin of the Pamir, east of the KES, moves northward together with the Tarim block, whereas the eastern Pamir west of the KES moves with a slight angle NNW toward the Tien Shan [Yang et al., 2008; Zubovich et al., 2010]. The KES therefore accommodates slight variations in the motion of crustal blocks, and we hypothesize that it forms a decoupling horizon. Although GPS data used to derive crustal velocities are sparse, existing data also suggest a gradient of active crustal convergence along the northern Pamir deformation front. Several studies indicate that high (13–15 mm yr−1) north-south convergence rates between the northern Pamir and the southern Tien Shan are mainly accommodated along the MPT in the western and central Pamir (see transect f in the work of Zubovich et al. [2010] or Figure 1b in the study by Li et al. [2012]). However, east of 75°E, between the East Pamir/Tarim Basin and the southern Tien Shan, only ~7 mm yr−1 of crustal shortening are mainly accommodated along the Pamir frontal thrust [Zubovich et al., 2010]; an additional 13–15 mm yr−1 of shortening is accommodated throughout the Tien Shan at this longitude (see transect a in the work of Zubovich et al. [2010]). Furthermore, the GPS data show that the western and central Pamir regions diverge from the East Pamir/Tarim by 5–8 mm yr−1 [Zubovich et al., 2010], which is accommodated by crustal extension in the north-central, south central [Stübner et al., submitted, 2013b], and eastern Pamir. This agrees well with active seismicity indicating extension in these regions [Burtman and Molnar, 1993; Strecker et al., 1995]. The folding of Mesozoic and Cenozoic sedimentary rocks along the western edge of the Tarim Basin [Sobel and Dumitru, 1997; Sobel, 1999; Cowgill, 2010] implies convergence perpendicular to the eastern margin of the Pamir during the Neogene, but at present this convergence must be slow compared with the rate of east-west divergence across the wider Pamir salient. The Pamir is presently colliding with the Tien Shan [Leith, 1982; Hamburger et al., 1992]; presumably the onset of this collision was marked by the introduction of thicker, lower-density crust into the subduction zone [Hamburger et al., 1992; Sobel et al., 2011]. However, it appears that this collision has not caused a significant reduction in the northward motion of the Pamir. Rather, shortening has shifted northward into the Tien Shan, as suggested by an enhanced period of uplift observed throughout the Tien Shan beginning at ~10 Ma [e.g., Sobel et al., 2006]. This uplift pattern indicates higher crustal rigidity and spatially disparate and diachronous reactivation of inherited zones of crustal weakness to accommodate north-south convergence [Hilley et al., 2005; Macaulay et al., ]. Therefore, with the onset of the Pamir-Tien Shan collision, the crust of the Pamir may have increasingly started to diverge toward the less rigid Tadjik depression [e.g., Leith, 1982] in the west. This assessment agrees well with previous studies from the northeastern Pamir, which suggest that east-west crustal extension across the KES was ongoing by at least 7 Ma [Robinson et al., 2004]. Folding and thrusting within the Tadjik depression confirm that the northwestward transpressive deformation has been ongoing there as well for several million years [e.g., Thomas et al., 1994]. We further hypothesize that E-W extension is superimposed on secondary crustal processes, where excessively thickened crust in the vicinity of the northeast Pamir domes is coevally extruded vertically, forming the Kongur Shan massif. However, verifying this hypothesis requires additional kinematic and thermochronologic analyses from the central and western Pamir.

7 Conclusions

[48] The KES, along with the Muztagh Ata and Kongur Shan gneiss domes in its footwall and a high-elevation intermontane basin in its hanging wall are the first-order tectonogeomorphic features related to the late Miocene-Pliocene evolution of the northeast Pamir in the northwestern corner of the India-Eurasia collision zone. Building on previous work and integrating new ZHe, AFT, and 40Ar/39Ar data with one-dimensional thermal modeling, we generate new constraints on the tectonic evolution of the KES and the Kongur Shan and Muztagh Ata gneiss domes. Our results document significant temporal and spatial variations during the evolution of this fault system and its associated crustal-scale domes. We present a new model explaining why active east-west extension in the Pamir is localized along the KES and highlight its role in the regional tectonic evolution. In addition, we particularly address the question of why extension is so prominently localized along the eastern margin of the Pamir.

  1. [49] Our new data support a successive rather than synchronous growth of the Muztagh Ata and Kongur Shan domal massifs. Based on regional structural and geomorphic relationships and our thermochronological results, we favor two entirely separate events, although the data do not unambiguously prove this. Much of the southern flank of the Muztagh Ata massif was exhumed prior to ~8 Ma, most likely during north-south extension associated with exhumation of the Shatput dome, as suggested by Robinson et al. [2007]. Furthermore, >5 Ma old ZHe and AFT cooling ages suggest limited cooling and fault displacement along the southern fault segments of the KES adjacent to the Muztagh Ata dome since ~5 Ma. However, the core of the dome appears to experience uplift and continued crustal warping, as suggested by moderately high topographic relief (1–2 km, see Figure 6) and exhumation rates of ~1 ± 0.5 mm a−1.

  2. [50] In contrast to Muztagh Ata, the Kongur Shan crustal dome is characterized by late Miocene to recent rapid cooling, high topographic relief (>4 km), and tectonically driven exhumation of deep-seated rocks in the footwall of the KES. Age-elevation transects along the footwall detachment of the massif yield very young cooling ages for all applied thermochronometers, suggesting ongoing exhumation with rates ranging from >2–3 mm a−1.

  3. [51] New data along the KATT, along the northwestern part of the KES, elucidate the faulting history of the KES from the latest Miocene onward. However, as only ZHe and AFT thermochronometers yield reset late Miocene-Pliocene cooling ages and moderately high topographic relief (1–2 km) in the footwall, exhumation is less significant and the geothermal gradient is probably less steep along this part of the fault system. This is in excellent agreement with previous data suggesting slow cooling until 7 Ma, with a subsequent increase in cooling rate since [Robinson et al., 2004].

  4. [52] Over the last ~7–5 Ma, the northeastern Pamir appears to have been decoupled from transpressional deformation in the central and western sectors of the orogen and instead is undergoing transtensional deformation and east-west extension, as seen by the evolution of the central-northern Pamir graben system in the vicinity of Lake Karakul [Strecker et al., 1995; Amidon and Hynek, 2010], formation of the KES, and the slowdown of the KYTS along the eastern Pamir margin [Sobel et al., 2011]. We hypothesize that the existence of a crustal tear defining the lateral edge of the subducting slab [Sobel et al., in press] beneath the Pamir allows for greater heating along the eastern margin of the Pamir, providing a viable explanation for localization of extensional deformation along the KES and decoupling between the central/western Pamir and eastern Pamir/Tarim basin.

Acknowledgments

[53] We thank Alex Robinson and Mike Oskin for their constructive and helpful reviews that significantly improved the manuscript. We thank AE Lothar Ratschbacher for his constructive comments and suggestions and for introducing R.T. to the geology of central Pamir domes during joint fieldwork in the Tadjik Pamir in 2010. Funding was provided by the DFG Leibniz Center for Earth Surface and Climate Studies at Potsdam University (DFG grant STR 373/19-1), a grant to M. Strecker and L. Schoenbohm by the German Research Foundation (DFG grant STR 373/20-1), as well as grants to J. Chen by the International Science and Technology Cooperation Program of China (2008DFA20860), the National Science Foundation of China (41272195), and the SKLED (grant LED2010A04). Postdoctoral funding to R. Thiede by DFG (TH 1317/1-1 and 5–1) is also acknowledged.

Ancillary